While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited singlet state is created when excitons formed in an OLED device transfer their energy to the excited state of the dopant. However, it is generally believed that only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplet, which cannot readily transfer their energy to the singlet-excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant if it has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive, it can produce light by phosphorescence. In many cases, singlet excitons can also transfer their energy to lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission.
Singlet and triplet states, and fluorescence, phosphorescence, and intersystem crossing are discussed in J. G. Calvert and J. N. Pitts, Jr., Photochemistry (Wiley, New York, 1966). Emission from triplet states is generally very weak for most organic compounds because the transition from triplet-excited state to singlet ground state is spin-forbidden. However, it is possible for compounds with states possessing a strong spin-orbit coupling interaction to emit strongly from triplet-excited states to the singlet ground state (phosphorescence). One such strongly phosphorescent compound is fac-tris(2-phenyl-pyridinato-N^C-)Iridium(III) (Ir(ppy)3) that emits green light (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985), M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Güdel, M. Fortsch, and H.-B. Bürgi, Inorg. Chem., 33, 545 (1994)). Organic electroluminescent devices having high efficiency have been demonstrated with Ir(ppy)3 as the phosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys., 38, L1502 (1999)). Additional disclosures of phosphorescent materials and organic electroluminescent devices employing these materials are found in U.S. Pat. No. 6,303,238 B1, WO 00/57676, WO 00/70655 and WO 01/41512 A1.
There is a continuing need to develop new phosphorescent materials for improved stability and to provide a wide range of hues. S. Seo and co-workers, SID 05 Digest, 806 (2005), Y. Chi et al., Inorg. Chem., 44, 1344 (2005), C. Chen et al., Adv. Funct. Mater., 14, 1221 (2004), and C. Cheng et al., Adv. Mater., 15, 224 (2003) describe iridium complexes that include two C^N-cyclometallated quinazoline ligands and one ancillary ligand. The ancillary ligand is an anionic bidentate ligand that does not provide a carbon bonded to Ir. Although these materials are reported to have interesting properties it is generally the case that bis-cyclometallated metal complexes, although easier to synthesize, are less stable relative to tris-C^N-cyclometallated metal complexes.
Fujii et al, U.S. 2005/0191527 describes organometallic compounds with quinoxaline ligands including tris-C^N-cyclometallated complexes of iridium. Mishima et al., U.S. 2005/0191519 describes triplet emitters with various heterocyclic ligands. Some of these complexes are also tris-C^N-cyclometallated complexes of iridium, although none of the tris-C^N-cyclometallated complexes include a quinazoline ligand. However, many of the materials described in these disclosures emit at wavelengths that are too deep to be useful in a practical OLED device.
The nature of the host material is also critical to get good performance from the phosphorescent emitter. For example, U.S. Ser. No. 10/945,337 and U.S. Ser. No. 10/945,338 filed Sep. 20, 2004 (both now abandoned), and U.S. Ser. No. 11/015,929 and U.S. Ser. No. 11/016,134 filed Dec. 17, 2004 (now U.S. Pat. No. 7,767,316 and U.S. Pat. No. 7,579,090, respectively) describe an EL device in which the light emitting layer includes a hole-transporting compound, certain aluminum chelate materials, and a light-emitting phosphorescent compound. U.S. Ser. No. 11/214,176 filed Aug. 29, 2005 (published as US 2007/0048544), describes an EL device in which the light emitting layer includes a hole-transporting compound, certain gallium chelate materials, and a light-emitting phosphorescent compound.
Notwithstanding these developments, there remains a need for new phosphorescent materials to provide useful emission and stability attributes